Many different thread cutting tools are available for cutting screw threads on a member. For example, standard threading taps have a thread cutting portion usually consisting of a plurality of thread cutting lands spaced by elongated flutes and a shank portion for engagement with a power tool. Such taps are generally used to form internal screw threads in a hole that has been prepared in a connector. Standard thread cutting dies are known for cutting external threads on a connector such as, for example, a bolt. Such dies usually consist of a cutting head with internal thread cutting lands spaced by elongated flutes and may be driven by a power tool. However, these thread cutting tools may not be suitable for cutting screw threads in some softer materials and are limited to cutting only a single surface.
For example, it is difficult to form screw threads in soft metals, such as aluminum and brass, and soft plastics, such as delron and nylon, and other light materials. This is true because these types of materials generally cannot withstand the heat generated by the thread cutting process. The heat generated by the thread cutting process causes the materials to deform or melt.
Even if these soft materials can withstand the thread cutting process to receive threads, it is often difficult to obtain a strong threaded connection between two connector members because there is typically only a single threaded surface on each member through which the connection is made. For example, a bolt has only an external threaded surface for mating engagement with a threaded hole having only an internal threaded surface. Such threaded connections are unsuitable in applications that require a great amount of force because such force may cause the threads to be stripped due to the soft nature of the material.
Another reason strong threaded connections are difficult to obtain in soft materials is because such material may not withstand a substantial amount of torque. If too much torque is applied and the parts are over-tightened, the threads may be stripped due to the soft nature of the material.
In order to provide a strong threaded connection, some threaded connectors are provided with two sets of threads. For example, such connectors may have concentric opposed threaded surfaces that are adapted to mate with concentric opposed threaded surfaces on another member. One type of such a connector may be a pipe or cylinder having threads on both internal and external surfaces for connection with another member having a threaded annular opening with mating internal and external threads. Thus, each part has two threaded surfaces. Such a threaded connection can withstand a greater amount of force because of the increased number of threads. Such threaded connections can also withstand a greater amount of torque to be applied to the parts to obtain a tighter connection.
However, one problem with such concentric threaded connections is that they are difficult and time consuming to manufacture. For example, a two-step process must generally be performed to thread the opposed concentric surfaces of the pipe. First, one surface of the pipe must be threaded with one tool such as a tap to provide internal threads. Then, the opposed concentric surface must be threaded with another tool such as a die to form external threads on the pipe.
In order to form a mating threaded annular opening in a member to which the pipe may be connected, a hole must first be drilled into the member. This hole must then be threaded with a conventional tap to form the internal screw threads on the walls of the hole. Then, an upstanding post or shaft must be manufactured and threaded with a conventional die to form the external screw threads. This post must then be bolted or otherwise secured to the member at a location centrally of the hole. Thus, this member requires separate parts that must undergo individual threading operations. This construction increases the likelihood of a loose connection or what is commonly known as "slop". Thus, there is no known thread cutting tool that simultaneously forms screw threads on concentric opposed surfaces of a threaded connector.